Semiconductor integrated circuit device and method of controlling the same

ABSTRACT

Sub bit lines are connected to associated main bit lines, which are common to a plurality of memory blocks, through respective first MOS field effect transistors. The main bit lines are grounded through respective second MOS field effect transistors. When the first and second MOS field effect transistors are turned on, charges accumulated in drain regions of nonvolatile memory cells are extracted via the first MOS field effect transistors, the main bit lines and the second MOS field effect transistors.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a semiconductor integrated circuit device and the method of controlling the same.

[0002] An EEPROM (electrically erasable programmable read-only memory), one of different kinds of nonvolatile memories, has a plurality of memory cells. A known memory Ad cell consists of a floating gate field effect transistor (FET) with a double-layer electrode structure composed of a source region, a drain region, a channel region, a floating gate electrode provided on the channel region via an insulating film, and a control gate electrode provided on the floating gate electrode via an insulating film. A control signal is sent to the control gate electrode via a word line. As a structure of the word line, a structure constituted by a main word line and a sub word line connected to the main word line via a switching element is known. Such a structure is employed in a conventional flash memory, in which a large amount of currents flow into memory cells during a read or write operation. This is because, when a structure composed of a bit line and a sub bit line is employed in the flash memory, a voltage across the switching element for connecting the main bit line and the sub bit line falls markedly and this may result in misreading of the memory cells or an increased load on a circuit for generating a write voltage.

[0003] The flash memory has a plurality of memory blocks with memory cells arranged in a matrix therein. Each of the memory blocks is provided with a bit line decoder and bit lines, independently of the other memory blocks. Information is read from the flash memory having such a configuration as follows.

[0004] First, a switching element connecting a main word line and a sub word line is turned on to select the main word line and the sub word line, and then a voltage required for a read operation is applied to the sub word line through the main word line. Subsequently, a predetermined voltage is applied to a selected bit line to select a memory cell. Then, a current flowing through the selected memory cell is compared with a predetermined reference value by a differential amplifier, and a result determined based on the comparison is output as data.

[0005] At this time, when a charge remains in drains of unselected memory cells commonly connected to the sub word line, that is, memory cells for which the associated word line is selected and of which the sources have a low potential, the charge accumulated in the drain is pulled out, or extracted to the source side. In the flash memory, since memory cells in the same memory block usually have their sources commonly connected, currents to the sources in these unselected memory cells raise a source potential and may result in misreading.

[0006] Furthermore, since the charge in the drain is pulled to the source side in the unselected memory cells, hot electrons, that is, high-energy electrons are generated, and the electrons are injected into the floating gate electrodes of the unselected memory cells. Therefore, the threshold value of the unselected memory cells may rise.

[0007] Therefore, in general, the bit lines connected to the drains of the memory cells are connected to a discharge circuit via a switching element so that the charge in the drains of the memory cells is extracted via the bit lines when the flash memory is in a standby state, that is, a non-access state.

[0008] Meanwhile, in the conventional flash memory, when electrons are extracted from a floating gate electrode of a memory cell (an erase operation is assumed here), a method of applying a negative voltage to a word line is employed in general. In this case, the following problems occur in a flash memory having main word lines and sub word lines, the latter being provided in each memory block independently of the other memory blocks.

[0009] The switching element connecting the main word line and the sub word line needs to transmit a positive voltage during read and write operations and a negative voltage during an erase operation. Therefore, voltage control of the switching element becomes complicated and a control circuit provided to apply the positive voltage and the negative voltage to the switching element disadvantageously enlarges the memory.

[0010] An example of a semiconductor integrated circuit device having main word lines and sub word lines like the above-described flash memory is shown in FIG. 5. The semiconductor integrated circuit device has a memory cell region M made up of a plurality of memory blocks MB₀, . . . , MB_(X) and main word lines WLM₀, . . . , WLM_(n) commonly connected to the plurality of memory block MB₀, . . . , MB_(X). Each memory block MB₀, . . . , MB_(X) has a plurality of memory cells MC₀₀, . . . , MC_(nm) arranged in a matrix. Each memory cell MC₀₀, . . . , MC_(nm), consists of a floating gate type field effect transistor having a control electrode and a floating electrode. Furthermore, in each of the memory blocks MB₀, . . . , MB_(X), drain regions of the memory cells in the same column are commonly connected by an associated bit line BL₀, . . . , or BL_(m), while control gate electrodes of the memory cells in the same row are commonly connected by a sub word line WLS₀, . . . , or WLS_(n). Furthermore, P-type MOS (Metal Oxide Semiconductor) field effect transistors LWS₀₀, . . . , LWS_(Xn) for selecting sub word lines WLS₀, . . . , WLS_(n) are provided in each of the memory blocks MB₀, . . . , MB_(X). The P-type MOS field effect transistors LWS₀₀, . . . , LWS_(Xn) of each memory block are connected to a sub word line selecting circuit 100 via an associated memory block selecting gate line BS₀, . . . , or BS_(X) and to a voltage switching circuit 200 for switching between a back bias high voltage and a voltage VSS via an associated back bias supply line NW₀, . . . , or NW_(X). Furthermore, to pull out a charge accumulated in the bit line BL₀, . . . , BL_(m), the bit line BL₀, . . . , BL_(m) is grounded via a MOS field effect transistor DC₀, . . . , DC_(m). The MOS field effect transistor DC₀, . . . , DC_(m) is controlled by a discharge selecting gate line DDC₀, . . . , DDC_(m). Furthermore, the main word line WLM₀, . . . , WLM_(n) and the sub word line WLS₀, . . . , WLS_(n) are connected via the P-type MOS field effect transistor LWS₀₀, . . . , LWS_(Xn). A voltage is supplied from a main word line decoder MWD to the main word line WLM₀, . . . , WLM_(n). Furthermore, in the sub word line selecting circuit 100 from which a voltage is supplied to a P-type MOS field effect transistor LWS₀₀, . . . , LWS_(Xn), control for generating a negative voltage required depending on the operation is performed. As is obvious, since the field effect transistors DC₀, . . . , DC_(m), the sub word line selecting circuit 100 and the voltage switching circuit 200 are required in each memory block MB₀, . . . , MB_(X), the circuit size becomes large.

[0011] Hereafter, voltage control during a read operation in the semiconductor integrated circuit device will be explained below with reference to FIG. 6. In FIG. 6, it is assumed that a memory cell MC₀₀ in the memory block MB₀ is selected.

[0012] In a read operation, as shown in FIG. 6, a voltage of about 5 V is applied to the control gate electrode of the memory cell MC₀₀, while a voltage of about 1 V is applied to the drain region of the memory cell MC₀₀. At this time, a voltage to be applied to the control gate electrodes of unselected memory cells MC₁₀, . . . , MC_(1m); . . . ; MC_(n0), . . . , MC_(nm) in the memory block MB₀ needs to be set at 0 V. Therefore, a voltage of −5 V is applied to the memory block selecting gate line BS₀, and the main word lines WLM₁-WLM_(n) at 0 V are electrically connected to the associated sub word line WLS₁-WLS_(n).

[0013] Voltage control during a write operation in the semiconductor integrated circuit device is explained below with reference to FIG. 7. In FIG. 7, it is assumed that a memory cell MC₀₀ in the memory block MB₀ is selected.

[0014] During a write operation, as shown in FIG. 7, a voltage of about 10 V is applied to the control gate electrode of the memory cell MC₀₀, while a voltage of 4 to 5 V is applied to the drain region of the memory cell MC₀₀. At this time, a voltage to be applied to the control gate electrodes of unselected memory cells MC₁₀, . . . , MC_(1m); . . . ; MC_(n0), . . . , MC_(nm) in the memory block MB₀ needs to be set at 0 V. Therefore, a voltage of the memory block selecting gate line BS₀ must be a negative voltage. Furthermore, a voltage of 10 V must not be applied to the sub word lines WLS₀, . . . , WLS_(n) in the unselected memory blocks MB₁, . . . , MB_(X) so that write reliability is ensured. That is, the voltage of the control gate electrodes of the memory cells MC₀₀, . . . , MC_(nm) in the unselected memory blocks MB₁, . . . , MB_(X) needs to be 0 V. Therefore, a voltage of about 10 V is applied to the gate electrodes of the P-type MOS field effect transistors LWS₁₀-LWS_(xn) in the unselected memory blocks MB₁, . . . , MB_(X). Meanwhile, when a voltage of about −5 V similar to that of the read operation is applied to the gate electrode of the P-type MOS field effect transistor LWS₀₀, . . . , LWS_(0n) in the selected memory block MB₀, a voltage across the source and gate electrode of the P-type MOS field effect transistor LWS₀₀ exceeds the withstand or breakdown voltage, and the P-type MOS field effect transistor LWS₀₀ may be damaged. Therefore, during the write operation, the gate voltage of the P-type MOS field effect transistor LWS₀₀, . . . , LWS_(0n) in the selected memory block MB₀ is generally set at a voltage smaller than that of the read operation, for example, −2 V.

[0015] As described above, in the case of a semiconductor integrated circuit device having main word lines WLM₀, . . . , WLM_(n) and sub word lines WLS₀, . . . , WLS_(n), voltage control of gate electrodes of switching elements LWS₀₀, . . . , LWS_(Xn) or the like is essential during each of read and write operations. Therefore, a back bias control circuit of switching elements LWS₀₀, . . . , LWS_(Xn) selecting the sub word lines WLS₀, . . . , WLS_(n) and a circuit for controlling gate voltages of switching elements LWS₀₀, . . . , LWS_(Xn) are required. As a result, the control circuit becomes complicated and the peripheral circuit is enlarged, which may result in a larger chip size.

[0016] Meanwhile, with recent development in fine processing technology, a lower operating current for a memory cell has been achieved. Consequently, memory array configuration having main bit lines and sub bit lines has become employable in a semiconductor integrated circuit device. Since each memory block has word lines independently of the other memory blocks in the semiconductor integrated circuit device having main bit lines and sub bit lines, the switching elements requiring complicated voltage control become unnecessary. Furthermore, since the wiring capacitance of the word line is decreased, time required for reading or writing data from/to the memory cell can be shortened.

[0017] However, even in the semiconductor integrated circuit device having main bit lines and sub bit lines, a switching element for extracting a charge accumulated in the drain region of the memory cell at the time of standby (at the time of non-access) needs to be disposed in each memory block. As a result, a circuit for controlling the switching element is required in each memory block, and thus a problem arises that the circuit size is increased.

[0018] Examples of the semiconductor integrated circuit device having main bit lines and sub bit lines described above include the one disclosed in Japanese Patent Laid-Open Publication No. 9-153559. The semiconductor integrated circuit device described in this Japanese Patent Laid-Open Publication is provided with elements for pulling a charge accumulated in the drain regions of memory cells without passing through the memory cells and has a path for eliminating the charge. However, according to the semiconductor integrated circuit device of the Japanese Patent Laid-Open Publication No. 9-153559, since the elements for pulling the charge and a circuit for controlling the elements are provided in each of a plurality of memory blocks, a problem arises that a circuit occupation area around the memory block is increased.

SUMMARY OF THE INVENTION

[0019] Accordingly, the object of the present invention is to provide a semiconductor integrated circuit device which is allowed to have a reduced circuit occupation area, and a method of controlling the same.

[0020] A semiconductor integrated circuit device according to the present invention includes a plurality of memory blocks. Each memory block includes:

[0021] a plurality of nonvolatile memory cells arranged in a matrix form, each nonvolatile memory cell having a gate region, a source region and a drain region;

[0022] word lines each of which is commonly connected to the gate regions of the nonvolatile memory cells in the same row;

[0023] a source line which is commonly connected to the source regions of the nonvolatile memory cells;

[0024] sub bit lines each of which is commonly connected to the drain regions of the nonvolatile memory cells in the same column; and

[0025] first switching elements for selecting the associated sub bit lines,

[0026] The plurality of memory blocks being arranged in a column direction, and the semiconductor integrated circuit device further includes:

[0027] main bit lines which are common to the plurality of memory blocks and to which the sub bit lines in each memory block are connected via the respective first switching elements; and

[0028] at least one second switching element for extracting charges accumulated in drain regions of the nonvolatile memory cells via the first switching elements and the main bit lines.

[0029] According to the semiconductor integrated circuit device having the above configuration, when the first and second switching elements are turned on, charges accumulated in the drain regions of the nonvolatile memory cells are extracted or pulled out via the first switching elements and the main bit line. Since the charges in the drain regions of the nonvolatile memory cells are extracted therefrom via the first switching elements and the main bit lines, no switching elements for extracting the charges from the drain regions of the nonvolatile memory cells need to be provided on the sub bit lines. That is, no second switching elements for extracting the charges need to be provided in each memory block. Therefore, no circuits for controlling the second switching elements for extracting the charges need to be provided for each memory block, either. Thus the circuit occupation area of the semiconductor integrated circuit device of the present invention can be reduced.

[0030] Furthermore, because each of the main bit lines is commonly used by the plurality of memory blocks, control for extracting charges does not need to be performed in each memory block. Therefore, the control for extracting the charges from the drain regions of the nonvolatile memory cells is simplified.

[0031] Each memory cell may be constituted of a floating gate type field effect transistor.

[0032] In one embodiment, one second switching element is provided for one main bit line. Thus, a charge can be extracted every main bit line by controlling the second switching elements.

[0033] In one embodiment, the main bit lines are grounded via the at least one second switching element. Thus, the charge accumulated in the drain region of the nonvolatile memory cell can be allowed to escape to the ground.

[0034] The first switching elements in one memory block may be connected to a common signal line.

[0035] In one embodiment, each of the first switching elements is constituted of a MOS field effect transistor through which the sub bit lines are connected to the associated bit lines. Also, the gates of the MOS field effect transistors in one memory block are commonly connected to a memory block selecting gate line.

[0036] The present invention also provides a method of controlling the semiconductor integrated circuit device with the above arrangement. In the controlling method, upon completion of at least one of read, write and erase operations of selected nonvolatile memory cells in the memory blocks, the first switching elements and the at least one second switching element are turned on.

[0037] With this controlling method, occurrence of hot electrons can be prevented by extracting the charges in the drain regions of the nonvolatile memory cells via the first switching elements, the main bit lines and the at least one second switching element.

[0038] The present invention provides another method of controlling the semiconductor integrated circuit device with the above arrangement. In this controlling method, upon completion of each of read, write and erase operations of selected nonvolatile memory cells in the memory blocks, the first switching elements are turned on.

[0039] According to this controlling method, since the first switching elements are in an on state before and after the read, write and erase operations of the selected nonvolatile memory cells in the memory blocks, charges in the drain regions of the nonvolatile memory cells are extracted only by turning on the second switching element. Therefore, control for extracting the charges before and after the read, write and erase operations of the selected nonvolatile memory cells is simplified.

[0040] Furthermore, the present invention provides a still another method of controlling the semiconductor integrated circuit device with the above configuration. In the controlling method, the first switching elements and the at least one second switching element are turned on in a standby state of the device to thereby extract charges accumulated in the drain regions of the nonvolatile memory cells.

[0041] With this controlling method, occurrence of hot electrons after the device is released from the standby state can be prevented.

[0042] Other objects, features and advantages of the present invention will be obvious from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

[0044]FIG. 1 is a circuit diagram showing a substantial portion of a semiconductor integrated circuit device according to one embodiment of the invention;

[0045]FIG. 2 is a circuit diagram for explaining a standby state of the semiconductor integrated circuit device;

[0046]FIG. 3 is a circuit diagram for explaining a read operation of the semiconductor integrated circuit device;

[0047]FIG. 4 is a circuit diagram for explaining a write operation of the semiconductor integrated circuit device;

[0048]FIG. 5 is a circuit diagram showing a substantial portion of the conventional semiconductor integrated circuit;

[0049]FIG. 6 is a circuit diagram for explaining a read operation of the conventional semiconductor integrated circuit device; and

[0050]FIG. 7 is a circuit diagram for explaining a write operation of the conventional semiconductor integrated circuit device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] An embodiment of the semiconductor integrated circuit device and a method of controlling the same according to the present invention will be described in detail below with reference to accompanying drawings.

[0052]FIG. 1 is a circuit diagram showing a substantial portion of a semiconductor integrated circuit device according to one embodiment of the present invention.

[0053] As shown in FIG. 1, the semiconductor integrated circuit device has a memory cell region M constituted of a plurality of memory blocks MB₀, . . . , MB_(X). Although not shown, further memory cell regions M are arrayed in a horizontal direction, and word lines WL₀₀, . . . , WL_(Xn) are commonly used. In each of the memory blocks MB₀, . . . , MB_(X), a plurality of nonvolatile memory cells MC₀₀, . . . , MC_(Nn) are arranged in a matrix, namely in rows and columns. Each of the nonvolatile memory cells MC₀₀, . . . , MC_(Nn) is a floating gate type field effect transistor having a double-layer electrode structure composed of a source region, a drain region, a channel region, a floating gate electrode provided on the channel region via an insulating film and a control gate electrode provided on the floating gate electrode via an insulating film although not shown in the figure. Furthermore, each of the memory blocks MB₀, . . . , MB_(X) has word lines WL₀₀, . . . , WL_(Xn) to which gate regions of the nonvolatile memory cells MC₀₀ . . . MC_(N0); MC₀₁ . . . MC_(N1); . . . ; MC_(0n) . . . MC_(Nn) in the same row are commonly connected, source lines SL₀, . . . , SL_(X) to which source regions of the nonvolatile memory cells MC₀₀, . . . , MC_(Nn) are commonly connected, and sub bit lines BLS₀₀, . . . , BLS_(XN) to which drain regions of the nonvolatile memory cells MC₀₀ . . . MC_(0n); MC₁₀ . . . MC_(1n); . . . ; MC_(N0) . . . MC_(Nn) in the same column are commonly connected. Furthermore, the memory blocks MB₀, . . . , MB_(X) are provided with first MOS field effect transistors Q_(1S00), . . . , Q_(1SXN) as first switching elements for selecting each sub bit line BLS₀₀, . . . , BLS_(XN). Furthermore, the nonvolatile memory cells MC₀₀, . . . , MC_(Nn) are connected in parallel between the sub bit lines BLS₀₀, . . . , BLS_(XN) and the source lines SL₀, . . . , SL_(X).

[0054] Furthermore, the semiconductor integrated circuit device has main bit lines BLM₀, . . . , BLM_(N) commonly connected to a plurality of memory blocks MB₀, . . . , MB_(X) for each column.

[0055] The sub bit lines BLS₀₀, . . . , BLS_(XN) in each memory block MB₀, . . . , MB_(X) are electrically connected to the main bit lines BLM₀, . . . , BLM_(N) via the respective first MOS field effect transistors Q_(1S00), . . . , Q_(1SXN). Furthermore, second MOS field effect transistors Q_(2S0), . . . , Q_(2SN) are provided as second switching elements for each of the main bit lines BLM₀, . . . , BLM_(N), and one end of each main bit line BLM₀, . . . , BLM_(N) is electrically connected to a ground potential VSS via the respective second MOS field effect transistors Q_(2S0), . . . , Q_(2SN). Meanwhile, the other end of each main bit line BLM₀, . . . , BLM_(N) is electrically connected to a sense amplifier SA₀, . . . , SA_(N) via a third MOS field effect transistor Q_(SE0), . . . , Q_(SEN). The second MOS field effect transistor Q_(2S0), . . . , Q_(2SN) receives a signal from a discharge selecting gate line DDC₀, . . . , DDC_(N), and the third MOS field effect transistor Q_(SE0), . . . , Q_(SEN) receives a signal from a main bit line selecting gate line TR₀, . . . , TR_(N). Reference symbol SREF in FIG. 1 denotes a signal line for transmitting a reference voltage level for comparison when a nonvolatile memory cell MC₀₀, . . . , MC_(Nn) in a memory block MB₀, . . . , MB_(X) is read. This signal line SREF is connected to each sense amplifier SA₀, SA₁, . . . , SA_(N). Furthermore, the first MOS field effect transistors Q_(1S00), . . . , Q_(1SXN) are driven by the memory block selecting gate lines BS₀, . . . , BS_(X). The first MOS field effect transistors Q_(1S00), . . . , Q_(1SXN) in each memory block MB₀, . . . , MB_(X) are commonly connected to the associated memory block selecting gate line BS₀, . . . , BS_(X).

[0056] In FIG. 1, the subscript ‘X’ corresponds to a block number. That is, the number of the memory blocks MB₀, . . . , MB_(X) is (X+1). The subscript ‘n’ for the word lines WL₀₀, . . . , WL_(Xn) and ‘N’ for the main bit line BLM₀, . . . , BLM_(N) correspond to the numbers of those lines, respectively. That is, the number of the word lines WL₀₀, . . . , WL_(Xn) is (n+1) in each memory block MB₀, . . . , MB_(X) and (X+1)·(n+1) in the whole circuit, while the number of the main bit lines BLM₀, . . . , BLM_(N) is (N+1) in the whole circuit. Furthermore, the number of the nonvolatile memory cells MC₀₀, . . . , MC_(Nn) corresponds to the numbers of the word lines WL₀₀, . . . , WL_(Xn) and the main bit lines BLM₀, . . . , BLM_(N). Consequently, the number of the nonvolatile memory cells MC₀₀, . . . , MC_(Nn) is (N+1)·(n+1) in each memory block MB₀, . . . , MB_(X) and (X+1)·(N+1)·(n+1) in the whole circuit. It is noted that X, N and n are natural numbers. Furthermore, the number of the first MOS field effect transistors Q_(1S00), . . . , Q_(1SXN) connected to one main bit line BLM₀, . . . , BLM_(N) is (X+1).

[0057] According to the semiconductor integrated circuit device having the above configuration, when the first MOS field effect transistor Q_(1S00), . . . , Q_(1SXN) is turned on while the second MOS field effect transistor Q_(2S0), . . . , Q_(2SN) is turned on, a charge accumulated in a region between the nonvolatile memory cell MC₀₀, . . . , MC_(Nn) and the first MOS field effect transistor Q_(1S00), . . . , Q_(1SXN), that is, the drain region of the nonvolatile memory cell MC₀₀, . . . , MC_(Nn) is extracted via the first MOS field effect transistor Q_(1S00), . . . , Q_(1SXN) and the main bit line BLM₀, . . . , BLM_(N). Since the charge is extracted from the drain region of the nonvolatile memory cell MC₀₀, . . . , MC_(Nn) via the first MOS field effect transistor Q_(1S00), . . . , Q_(1SXN) and the main bit line BLM₀, . . . , BLM_(N), no specialized switching elements for extracting a charge in the drain regions of the nonvolatile memory cells MC₀₀, . . . , MC_(Nn) are provided on the sub bit lines BLS₀₀, . . . , BLS_(XN) in each memory block. Accordingly, a circuit for controlling such specialized switching elements for extracting a charge is not provided for each memory block MB₀, . . . , MB_(X), either. Thus, the circuit occupation area is smaller than the conventional circuit occupation area.

[0058] Furthermore, since the main bit line BLM₀, . . . , BLM_(N) is shared or commonly used by the plurality of memory blocks MB₀, . . . , MB_(X), control for extracting the charge does not need to be performed in each memory block MB₀, . . . , MB_(X). Therefore, the control for extracting the charge from the drain regions of the nonvolatile memory cells MC₀₀, . . . , MC_(Nn) is simplified.

[0059] Furthermore, since the second MOS field effect transistor Q_(2S0), . . . , Q_(2SN) is provided on each main bit line BLM₀, . . . , BLM_(N), a charge can be extracted from each main bit line BLM₀, . . . , BLM_(N) by controlling the second MOS field effect transistor Q_(2S0), . . . , Q_(2SN).

[0060] Furthermore, since the main bit line BLM₀, . . . , BLM_(N) is grounded via the second MOS field effect transistor Q_(2S0), . . . , Q_(2SN), a charge accumulated in the drain region of the nonvolatile memory cell MC₀₀, . . . , MC_(Nn) can be allowed to escape to the ground.

[0061] A read operation and a write operation of the semiconductor integrated circuit device will be explained below with reference to FIGS. 2, 3 and 4. In FIGS. 2, 3 and 4, only one of the main bit lines BLM₀, . . . , BLM_(N) is representatively shown. Therefore, the subscripts 0-N for the main bit lines are omitted from the reference numerals in FIGS. 2, 3 and 4.

[0062] First, the read operation of the semiconductor integrated circuit device will be explained.

[0063] Before the read operation is started, a voltage Vcc is applied to the memory block selecting gate lines BS₀, . . . , BS_(X), as shown in FIG. 2, to turn on all the first MOS field effect transistors Q_(1S0), . . . , Q_(1SX). Then, a voltage Vcc is applied to the discharge selecting gate line DDC to turn on the second MOS field effect transistor Q_(2S). A charge accumulated in the main bit line BLM and a parasitic capacitor C₀, . . . , C_(X) connected to the sub bit line BLS₀, . . . , BLS_(X) in each memory block MB₀, . . . , MB_(X) is discharged to a ground potential VSS.

[0064] Subsequently, when the read operation is performed, the sub bit line selecting signals BS₁, . . . , BS_(X) are set at 0 V in unselected memory blocks MB₁, . . . , MB_(X) to turn off their first MOS field effect transistors Q_(1S1), . . . , Q_(1SX). Then, the discharge selecting gate line DCC is set at 0 V to turn off the second MOS field effect transistor Q_(2S). On the other hand, in the selected memory block MB₀, a voltage of, for example, about 5 V is applied to a selected word line WL₀, while the other unselected word lines WL₁, . . . , WL_(n) are set at 0 V. At this time, a voltage of, for example, about 1 V is applied to the main bit line BLM, while the voltage of the source line SL is set at 0 V.

[0065] Then, after the read operation has been finished, the memory block selecting gate lines BS₀, . . . , BS_(X) are set at Vcc again as shown in FIG. 2 to turn on the first MOS field effect transistors Q_(1S0), . . . , Q_(1SX) in the memory blocks MB₀, . . . , MB_(X). Then, the discharge selecting gate line DDC is set at Vcc to turn on the second MOS field effect transistor Q_(2S). Consequently, a charge accumulated in the main bit line BLM and the sub bit line BLS₀, . . . , BLS_(X) is discharged to the ground potential VSS via the first MOS field effect transistor Q_(1S0), . . . , Q_(1SX), the main bit line BLM and the second MOS field effect transistor Q_(2S).

[0066] The write operation of the semiconductor integrated circuit device will be explained below.

[0067] Before the write operation is started, the memory block selecting gate lines BS₀, . . . , BS_(X) are set at a high voltage, that is, Vcc, as shown in FIG. 2, to turn on all the first MOS field effect transistors Q_(1S0), . . . , Q_(1SX), while the discharge selecting gate line DDC is set at Vcc to turn on the second MOS field effect transistor Q_(2S). Consequently, a charge accumulated in the main bit line BLM and a parasitic capacitor C₀, . . . , C_(X) connected to the sub bit line BLS₀, . . . , BLS_(X) in each memory block MB₀, . . . , MB_(X) is discharged to the ground potential VSS.

[0068] Subsequently, when the write operation is performed, memory block selecting gate lines BS₁, . . . , BS_(X) of unselected memory blocks MB₁, . . . , MB_(X) are set at 0 V, as shown in FIG. 4, to turn off their first MOS field effect transistors Q_(1S1), . . . , Q_(1SX). Furthermore, the discharge selecting gate line DDC is set at 0 V to turn off the second MOS field effect transistor Q_(2S). Then, a high voltage of about 12 V is applied to the first MOS field effect transistor Q_(1S0) of the selected memory block MB₀. Consequently, a voltage of about 4 to 5 V is transmitted from the selected main bit line BLM to the drain region of the memory cell MC₀, . . . , MC_(n) in the memory block MB₀. At this time, for example, a voltage of about 10 V is applied to a selected word line WL₀, while the other unselected word lines WL₁, . . . , WL_(X) are set at 0 V. Furthermore, the source line SL₀ of the memory block MB₀ is set at 0 V.

[0069] After the write operation has been finished, the memory block selecting gate lines BS₀, . . . , BS_(X) are set at a high voltage, that is, Vcc again, as shown in FIG. 2, to turn on the first MOS field effect transistors Q_(1S0), . . . , Q_(1SX) in each memory block MB₀, . . . , MB_(X). At this time, the discharge selecting gate line DDC is set at Vcc to turn on the second MOS field effect transistor Q_(2S). Consequently, a charge accumulated in the main bit line BLM and the sub bit line BLS₀, . . . , BLS_(X) is discharged to the ground potential VSS via the first MOS field effect transistor Q_(1S0), . . . , Q_(1SX), the main bit line BLM and the second MOS field effect transistor Q_(2S).

[0070] Because the charges are thus extracted from the drain regions of the nonvolatile memory cells MC₀, . . . MC_(n) before and after the read and write operations of the selected nonvolatile memory cell MC₀ in the memory block MB₀, occurrence of hot electrons is prevented.

[0071] In the above embodiment, the second MOS field effect transistors Q_(2S0), . . . , Q_(2SN) as the second switching elements are provided for the respective main bit lines BLM₀, . . . , BLM_(N). However, a plurality of main bit lines BLM₀, . . . , BLM_(N) may be connected to one common line and the second MOS field effect transistor Q_(2S) may be provided on this common line. In this case, a plurality of main bit lines BLM₀, . . . , BLM_(N) are controlled by one second MOS field effect transistor Q_(2S).

[0072] Furthermore, the sub bit lines BLS₀₀, . . . , BLS_(XN) are not grounded in the above embodiment, but may be grounded via a switching element. That is, the switching element for extracting a charge in the drain region of the nonvolatile memory cell MC₀₀, . . . , MC_(Nn) may be provided on the main bit line BLM₀, . . . , BLM_(N).

[0073] Furthermore, before and after the read and write operations of the selected nonvolatile memory cell MC₀ in the memory block MB₀, the first MOS field effect transistor Q_(1S0), . . . , Q_(1SX) may be turned on. In this case, a charge in the drain region of the nonvolatile memory cell MC₀, . . . MC_(n) can be extracted only by turning on the second MOS field effect transistor Q_(2S) before and after the read and write operations of the nonvolatile memory cell MC₀, control for extracting the charge is further simplified.

[0074] Furthermore, in the above embodiment, charges in the drain regions of the nonvolatile memory cells MC₀, . . . , MC_(n) are extracted before and after the read and write operations of the selected nonvolatile memory cell MC₀ in the memory block MB₀. However, the charges in the drain regions of the nonvolatile memory cells MC₀, . . . , MC_(n) may be extracted before and after an erase operation.

[0075] Furthermore, in the above embodiment, the first MOS field effect transistors Q_(1S0), . . . , Q_(1SX) and the second MOS field effect transistor Q_(2S) are turned on before and after the read and write operations of the selected nonvolatile memory cell MC₀ in the memory block MB₀. However, the first MOS field effect transistors Q_(1S0), . . . , Q_(1SX) and the second MOS field effect transistor Q_(2S) may be turned on after finishing at least one of the read, write and erase operations of the selected nonvolatile memory cell MC₀ in the memory block MB₀, . . . , MB_(X).

[0076] Furthermore, in a non-access state of the device, the first MOS field effect transistor Q_(1S0), . . . , Q_(1SX) and the second MOS field effect transistor Q_(2S) may be turned on to extract a charge accumulated in the drain region of the nonvolatile memory cell MC₀, . . . MC_(n).

[0077] Furthermore, the above has merely described a case where the present invention is applied to a flash memory technology, which is a field of the background of the present invention, but the present invention is not limited to this case, and, can be applied to other semiconductor integrated circuit device technology, such as a device in which EEPROM cells and a logic circuit are provided on the same semiconductor substrate. The present invention can be applied at least to a semiconductor integrated circuit device having the memory cell region M with the above-described configuration.

[0078] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

What is claimed is:
 1. A semiconductor integrated circuit device, comprising: a plurality of memory blocks, each memory block including: a plurality of nonvolatile memory cells arranged in a matrix form, each nonvolatile memory cell having a gate region, a source region and a drain region; word lines each of which is commonly connected to the gate regions of the nonvolatile memory cells in the same row; a source line which is commonly connected to the source regions of the nonvolatile memory cells; sub bit lines each of which is commonly connected to the drain regions of the nonvolatile memory cells in the same column; and first switching elements for selecting the associated sub bit lines; the plurality of memory blocks being arranged in a column direction, the semiconductor integrated circuit device further comprising: main bit lines which are common to the plurality of memory blocks and to which the sub bit lines in each memory block are connected via the respective first switching elements; and at least one second switching element for extracting charges accumulated in drain regions of the nonvolatile memory cells via the first switching elements and the main bit lines.
 2. The semiconductor integrated circuit device according to claim 1, wherein one second switching element is provided for one main bit line.
 3. The semiconductor integrated circuit device according to claim 1, wherein the main bit lines are grounded via the at least one second switching element.
 4. The semiconductor integrated circuit device according to claim 1, wherein each memory cell is constituted of a floating gate type field effect transistor.
 5. The semiconductor integrated circuit device according to claim 1, wherein the first switching elements in one memory block are connected to a common signal line.
 6. The semiconductor integrated circuit device according to claim 1, wherein each of the first switching elements is constituted of a MOS field effect transistor through which the sub bit lines are connected to the associated bit lines; and the gates of the MOS field effect transistors in one memory block are commonly connected to a memory block selecting gate line.
 7. A method of controlling the semiconductor integrated circuit device according to claim 1, wherein upon completion of at least one of read, write and erase operations of selected nonvolatile memory cells in the memory blocks, the first switching elements and the at least one second switching element are turned on.
 8. A method of controlling the semiconductor integrated circuit device according to claim 1, wherein upon completion of each of read, write and erase operations of selected nonvolatile memory cells in the memory blocks, the first switching elements are turned on.
 9. A method of controlling the semiconductor integrated circuit device according to claim 1, wherein the first switching elements and the at least one second switching element are turned on in a standby state of the device to thereby extract charges accumulated in the drain regions of the nonvolatile memory cells. 